Why are the lectures on www.edx.org different from the lectures on cs50.harvard.edu?
Which should I should take?
Let your decision be effected by this comparison. edX offers you course for free if you apply for honor code certificate and a threshold donation if you apply for a verified certificate.
The lectures on both of these resources are almost same(the syllabus and teaching patterns and etcetera). The only difference might be because of the two semesters as edX version was uploaded something around January and lasts till the end of December.